Tool induction elimination apparatus for electric control circuits



E. M. CAB-LENDER TOOL INDUCTION ELIMINATION APPARATUS FOR ELECTRIC CONTROL CIRCUITS Nov. 1, 19490 2 Sheets-Sheet 1 Filed Dec. 6, 1947 INVENTOR.

EDWIN M. CALLENDER ATTORNEY Filed Dec. 6, 1947 NQV, 1, 1949. M CALLENDER $486,552

TOOL INDUCTION ELIMINATION APPARATUS FOR ELECTRIC CONTROL CIRCUITS 2 Sheets-Sheet 2 .TNVENTOR.

EDWIN NLCALLENDER ATTORNEY Patented Nov. 1, 1949 TOOL INDUCTION ELIMINATION APPARATUS FOR ELECTRIC CONTROL CIRCUITS Edwin M. Callender, Cynwyd, Pa., assignor to The Budd Company, Philadelphia, Pa., a corporation of Pennsylvania Application December 6, 1947, Serial No. 790,192

13 Claims.

This invention relates to improved means for neutralizing the inductive effect of the parts of a tool, such as a welding tool, when utilized as part of the power supply circuit from a source of alternating current.

In my copending application, Serial No. 630,401, filed November 23, 1945, now Patent No. 2,472,043, dated May 31, 1949, there is shown and described an electrical control system which is dependent for functioning upon the change of load in a load circuit to which the control system is connected. The particular application of this circuit is to a resistance welder having electrodes and a tool yoke which in the operation of the apparatus develops an inductive voltage introducing an inaccuracy in the value of the control voltage. To overcome this inductive voltage, there is supplied in this circuit, an inductor subject to adjustment which by its action lessens or neutralizes the inductive effect of the tool and thus secures for the control system correct action.

In the present application, which is a continuation in part of the application above referred to, I employ the method and apparatus of said application and also modified and improved means for accomplishing the neutralization of tool inductive voltage.

The objects of this invention may be stated as follows: to provide effective means to neutralize inductive voltage in a signal circuit which develops from a tool connected to the signal circuit; to provide tool inductive voltage neutralizing means in which transformers are substantially eliminated; to provide neutralizing means for tool inductive voltages which may be set up either at the tool or at points removed from the tool; to provide a neutralizer for tool inductive voltages which is chiefly dependent upon voltage transmission as distinguished from power transmission; to provide a neutralizer of the mentioned type in which high excitation transmission is avoided; to provide a neutralizer in which the time lag in the operation of the. control voltage is reduced to a negligible value; to provide a neutralizer which may be made a part of and subject to operation of the current compensation circuit; and to provide neutralization means which, when adjusted, is fixed for all ordinary variable conditions that may occur in the operation of the tool.

In order to set out the features of the invention with clarity, the neutralizing circuit for the tool inductive voltage is shown and described as applied to a resistance welding circuit although 2 it is apparent that the circuit has utility in other relationships.

Reference is made to the following description based on the figures of drawing including:

Figure 1 showing a resistance welding circuit provided with combined current compensation and tool voltage neutralization;

Figure 2 showing a modification in which a transformer type neutralizer of tool voltage is employed as distinguished from a fast resistancetube neutralizer; and

Figure 3 showing a modification in which the neutralizer is structurally one with the tool itself.

Like parts bear similar numbers in the various views.

In the drawing, the circuit may be divided into various sections, lettered A, B, C, D, E, F and G, indicating, in general, different functions of the circuit. A indicates the usual power and timing controls. B denotes the power circuit of a welder including the tool structure and electrodes and power source. C indicates the neutralizing device for the tool inductive voltage. D denotes the current compensation circuit. E refers to the current rectifying and voltage amplifying circuit, F to the voltage control circuit, and G to the apparatus operated by the control circuit.

In greater detail in section B, there is indicated at I, a load comprising two overlying metal plates adapted to be welded together. These plates are placed between the electrodes 2 and 3, the electrodes, in turn, having connection through yoke arms 4 and 5 to the tool standard 6. Power is supplied the electrodes 2 and 3 from the A. C. source I of the A circuit through the ignitrons 8 and 9 and welding transformer 10, including the primary coil ll, secondary coil 12 and secondary conductors l3.

Conventional control of the power supply is diagrammatically indicated in the A section by the boxed units l5 and I6, designating respectively the power control and the timing control. The power control, in conjunction with the phase shift ll, selects the fraction of the half cycle of the supply current to be utilized at the weld and the timing control determines the number of cycles of power application for each weld. Details of these controls are more clearly shown in my copending application to which reference is made hereinabove.

The C, D, E and F circuit sections constitute the corrective, amplifying and control circuits which are effective for operating translating devices, such as a cutout switch in the G section, the control being subject to load change for operation. In the case of a resistance welding circuit, the change of resistance and voltage at the Weld point due to fusion is the initiating factor in bringing about operation of the control circuit. Circuit connections for the signal amplifying circuit E are made in electrodes 2 and 3 at points 28 and 2|, the conductors 22 passing through the yoke arms 4 and 5, and standard 6, extending from the tool, preferably intertwined as shown at 23, to reduce inductance, to two resistors 24 and 25, connected in series in the tool circuit 2I-23.

Resistor 25 is variable and is included in a shunt circuit from the current compensator circuit D, as will be more fully described hereinafter. The terminals 28 and 21 of resistor 24 are connected through bias resistors to grids 28 and 29 respectively of triode vacuum tubes 30 and 3|. The cathodes 32 and 33 of these tubes are connected together and to the midpoint of resistor 24 through a bias battery 34. The anodes 35 and 38 of the tubes 30 and 3l are connected together through two equal valued resistors 37 and 38 for which, in some uses, reactors may be used. A direct current source 39 is provided for the tube supply, connections being made at points 40 and 4i between the cathodes 32 and 33 and resistors 31 and 38 respectively. A smoothing capacitor 42 is also connected between points 49 and 4|.

The tube system 30-3l is of the push-pull type and is provided primarily for amplification. Obviously, additional stages of tube amplification may be utilized as desired, as indicated at 43.

The amplified current from the tube system 3ii3l is rectified and filtered by a full wave rectifier 44, resistor 44a and capacitors 44b and this rectified and smoothed current is led to resistor 45. A triode vacuum tube 46 and potentiometer resistor 4'! are connected in parallel around the resistor 45, establishing output terminals 48 and 49 for the E branch circuit.

The tube 48 controls the voltage of the E section output points 48 and 49 on either side of the resistor 41. This control is obtained by variation of the effective resistance of the tube through change of bias of grid 46a within the tube. This bias varies so as to increase the resistance and thereby the voltage between points 48 and 49 when, due to an increase of current in the load circuit as brought about by weld fusion, for example, the signal voltage is less than its normal value.

The means for compensating for current change in the load circuit is specifically illustrated in the branch circuit D. A current transformer 52] is connected to the secondary of the load circuit, one side of the current transformer secondary being arranged with taps 5i and movable contact arm 54a to secure a variation in the value of the transformation. A conductor 52 leads from tap contact arm 5Ia through a reactor coil 53 to a potentiometer 54 where connection is made with the other conductor 55 of the current transformer. A variable shunt resistor 55 is connected across the transformer to control the supply of power to the potentiometer, the resistance of resistor 56 being low compared with potentiometer 54 to insure proper control.

The output terminals 51 and 58 of the potentiometer 54 are connected to points 59 and 88 respectively, constituting inlet connections to a bridge rectifier [40. The outlet terminals 81 and 62 of this rectifier connect through conductors 63 and 84 respectively to a second potentiometer 55 to secure a desired direct current voltage for transmission to the control grid 15 of a pentode vacuum tube 19. Resistor 66 and capacitor 61 are employed for smoothing action on the rectified current flowing to the potentiometer 65. Leading from the inlet terminal 68 of potentiometer 65 a conductor 69 connects to the tube 10 at the cathode ll and suppresser grid thereof. From outlet terminal 12, which is positive in relation to terminal 68, a conductor 13 leads through a constant potential in the form of a battery 14 and a resistor to the control grid 75 of the tube 10. There is thus impressed between the control grid 15 and the cathode ll of tube 18, a voltage which is the sum of the constant voltage 14 and the rectified voltage due to the current in the load circuit and consequently, any change of current in the load circuit will be transmitted as a voltage change between points 12 and 68 to combine with the fixed voltage at the battery 74 and establish a variation in the bias of tube 10.

A voltage is applied across the anode l6 and cathode ll of tube 10 from a source of constant direct current ll, there being employed a variable contact 41a on the resistor 47 to secure a desired value of applied voltage. The circuit from the source 1? passes from the positive side thereof through the resistor 41 to point 48 and thence through resistors 78 and 79 to the anode 16 of the tube. The negative terminal of the source ll is connected by conductor 88 directly to the cathode H of the tube. Screen grid 8| is connected to point 82 intermediate the voltage point 48 and the resistor 78 and lying also in the connection between the cathode of tube 48 in the main signal circuit and point 48. The control grid 46a of tube 46, previously mentioned as controlling the voltage between points 48 and 49, is connected to the anode T8 of tube 10 through the resistor 19 and hence, the voltage of grid 45a is variable dependent upon the value of the current flowing through the tube 19 from source I It thus appears that variation of the voltage between points 68 and 12 of the D circuit impresses on the control grid 15 of tube 70, a bias variation which, in turn, produces a variation in the grid bias of tube 46 and thereby produces between points 48 and 49, a voltage variation, the direction of which is such as to compensate for the change in current in the load circuit. Following this change more specifically and assuming polarity of the rectifier I40 and battery 74 of the D circuit as indicated, an increase in current in the load circuit, which would normally result from weld fusion, would bring about an increase in voltage between points 53 and 12, and this would add to the voltage of battery 14- to increase the negative bias of control grid 15 of tube lil. This would increase the effective resistance of tube 18 and increase the voltage between the anode and cathode of this tube, thereby making the grid 46a of tube 46 more positive and reducing the voltage between points 48 and 49 in the signal circuit. Accordingly, as regards current change, points 48 and 49 are compensated.-

In describing the connections from the yoke of the weldin tool through conductors 23 and resistors 24 and 25, description or the neutralizing circuit C, employed for overcoming the inductive efiect of the tool yoke, was omitted for description at a later point. The neutralizing circuit comprises the reactor 53 in the conductor 52 of the D circuit, which reactor is provided with series of taps 53a and the movable contact 532) for transmission of modified reactance voltage to the variable resistor 25. The reactance is wound in such direction as to impress on resistor 25, a voltage opposite to that across resistor 24 due to voltage across the electrodes, i. e. 180 out of phase so that, since both resistors 24, 25 are in the signal circuit 22--23, a correction is imposed on the signal voltage to the extent of the opposing voltage across resistor 25. By appropriate adjustment of the reactance of coil 53 and the resistance of resistor 25 there may be obtained substantially exact neutralization of the inductive voltage of the tool in the signal circuit. This neutralization will be maintained so long as the surrouding conditions remain unchanged. Obviously, a new tool will require a new adjustment.

Hence, it now appears that the tool inductive voltage correction of the C circuit, coupled with the current change correction of the D circuit, permits establishment at output points 48 and 49 in the E circuit of a voltage value directly variable with the voltage across the electrodes.

The voltage control circuit F is connected directly to output points 48 and 49 of the E circuit. This circuit comprises a bridge normally unbalanced in one direction but which on balancing or unbalancing in the opposite direction, energizes mechanism of translating devices, such as relays, meters or motors. This bridge is formed of grid-controlled vacuum tubes 90 and 9| and resistors 92 and 93, each of these elements being connected in series with power inlet connections at junction points 94 and 95 (the latter being a potentiometer slider) and power outlet connections at junctions 96 and 91. The power inlet connections or junctions are supplied from an appropriate source of constant potential direct current as indicated at 98. Junction 95 is formed as a potentiometer slider in order to secure a variation of the balance of the bridge in preliminary settings, the bridge preferably being unbalanced for prevention of actuation of the translating devices prior to weld fusion.

The grid 90a of tube 90, forming one arm of the bridge, has connection through a resistor 99 to a point I intermediate resistors IM and I02, connected across the outlet terminals 48 and 49 of the amplifying signal circuit and consequently, the negative bias on the grid of tube 90 will vary with the signal output of the points 40 and 49. The grid 9Ia of tube 9| in another of the arms of the bridge, has connection through resistors I03 and I04 to point I05 in conductor I06, connected to the negative terminal of source 98. In order to secure proportional values, the ratio of the resistances of resistor I04 to I03 is similar to that of resistor I02 to resistor IOI. The gid 9Ia of this tube also has connection through the resistor I03, resistor I0! and capacitor I08 to conductor I06 at point I09. A rectifying circuit is provided to supply a charging current to the capacitor I09, including the rectifier III], the cathode of which has connection to points 40 and 49 through a potentiometer 1 I2, placed in paral lel with resistors I0! and I02 across points 48 and 49. The potentiometer slide II3 permits variation in the percentage of the voltage drop between points 48 and 49 which is applicable to the grid 9Ia of bridge tube 9I.

Points 95 and 9'! of the bridge constitute the output points of the F section, as well as the bridge, and function to control current flow in a gas tube I I4 in the G section through bias variation of the control grid II5 thereof, point 96 of the output having connection to the cathode II8 of this tube and point 9'! having connection to the control grid II5. Power is supplied to this gas tube from a source II! of constant potential direct current, the circuit including a variable resistor II8, a time delay relay coil 9' having a controllin resistor element I20, a relay coil I2I and a normally closed relay switch I22. Relay coil I2I when energized is adapted to open relay switch II I included in the timing control circuit I I2 of the B section.

The welding operation of the circuit is initiated by means of appropriate relay mechanism which may be indicated simply by a source of alternating current I23, having in series therewith a variable resistor I24, a manual switch I25 and a relay coil I26. The relay coil is adapted to close the relay switch I21 in the timing control circuit II2 of the A section so as to start the sequence of power supply and timing to bring about the welding cycle. This relay, also, when energized, closes relay switch I22 in the translating device circuit, including relay I2I, and additionally opens a normally closed relay switch I28 connected in shunt about the bridge capacitor I08. In the normally closed position of the switch I28, the capacitor is completely discharged but when opened, the charge on the capacitor is made effective on the grid of tube 9|. The association of the relay switches I28, I22 and I21 to the relay coil I 26 is indicated by broken lihes.

The complete operation of the circuit may now be detailed. On closure of the manual initiating switch I25, relay switch I2'I closes, instituting functioning of the timing and power control and startin a weld cycle at the electrodes. Closure of the initiating switch also opens relay switch I28, thus making effective on the grid of tube 9i, the initial voltage of the control system comparable to that of the electrodes prior to weld fusion. Relay switch I22 also is closed.

Prior to weld fusion, the voltage at the electrodes is transmitted, amplified and rectified to output points 48 and 49 with correction not only for current change if any, but also for tool yoke inductive voltage. From points 48 and 49, the initial voltage is transmitted to the F section bridge where a bias is established in the grids of tubes and 9| while at the same time, a charge is formed on the capacitor I08 equivalent to the initial voltage value.

The slider II3 of potentiometer H2 is adjusted so that a bridge output is negative at terminal 96, giving a small negative bias on the control grid II5 of gas tube II4.

At weld fusion, the resistance at the weld and, consequently, the voltage changes, and this change is made at once efiective at the grid of tube 90. However, because of the capacitor I08 in the circuit of the grid of tube 9I, the change of load voltage does not immediately take place in this tube and, therefore, the bridge is thrown out of balance in such direction as to develop a positive potential at point 91 and the attached control grid II5 of tube II4, whereby this tube is caused to strike and power relay I2I is energized to open the cutout switch I I I in the timing control circuit I I 2. Thereupon the power circuit to the electrodes is opened.

It is apparent that the complete circuit, as described, involves control of a translating device or mechanism by voltage control means and that to secure accurate control, it is essential that the voltage impressed on the voltage control means reflect accurately the voltage change at the electrodes, brought about by weld fusion. Theemployment of currentcorrection takes care of one-phase of this problem. However, the effect of inductance of the yoke in a welding tool is so pronounced as to introduce substantial modification in the transmitted voltage so that unless correction be made for this inductive voltage, the operation of the translating. device, such as the cutout, will be improperly timed. and the entire control mechanism will operate incorrectly.

By using a neutralizing circuit as described employing resistances and electronic tubes, it is seen that the time lag of neutralization is practically eliminated. Since control circuits for electrical apparatus, such as" welding tools, are usually complex, it becomes of considerable importance that time-consuming mechanism and circuits be eliminated as far as possible. This is apparent when it is understood that a welding cycle may as short as S or 10 cycles of a 60- cycle current. The described resistance tube arrangement for both tool inductive voltage neutralization and current compensation is a desirable solution of this problem.

Further, by combining the neutralizing and compensating circuits; they are made jointly subject to adjustment at tap switch m and compensation and neutralization thus constitutes a parallel operation which is continuously in step. The provision of the neutralizing resistor-reactor arrangement also avoids the necessity of placing a corrective inductive device directly on the tool itself as appears from a modification herein described and previously suggested in applicants copending application above referred to, the control apparatus being installed in the timer assembly where it is not subject to accidental maladjustment or injury,

There are adaptations of control circuits of the type described, where conditions warrant the use of transformers because of the easy amplification and stable operating characteristics of this apparatus. In the modified circuit, as shown in Fig, 2', transformers are used of low ratio transformation so as to reduce the reactance and improve the time constant factor.

Referring to Fig. 2, there is disclosed the B-C-D-E sections of the complete circuit, as shown in Fig. 1, it being understood that these circuit sections may be substituted as a unit for the same lettered sections of Fig. l.

The B section includes the tool with frame portions 5, 5 and 6 and. electrodes 2 and 3. The signal conductors 22-23 are connected to the primary Iiifl of a transformer I51 and also to the secondary I52 of transformer I53. The primary I54 of transformer I53 forms part of av branch circuit, including reactor 53, having taps 53a. and movable contact 53b to adjust for different values of reactance. The reactor forms a part also of the current compensation circuit, as will be more fully described hereinafter.

The secondary of transformer I5I establishes a potential across a resistor I55 and thereby impresses a control voltage on the control grid I56 of the tetrode vacuum tube I 5 1. This tube forms part of an amplifying circuit, including a direct current voltage source I5'Ia, bias resistor I58, capacitor I59, screen grid. I60, anode [GI and primary I62 of a transformer I63.

The amplifying transformer I63forms part also of a rectifier'circuit for the signal current. There is included in this circuit, a full wave vacuum rectifier tube I64 with anodes I65 connected to 8 the terminals of secondary I36 of transformer I63, filter elements including the reactor I61 andcapacitors I68, and the load resistance 45 having terminals I69 and H3. These'terminals, in turn, connect respectively to E output terminals 48 and 49, tenninals I'm-and 49 being directly connected and terminals I69 and 48 being'connected through the: vacuum tube 46 as in Fig. 1. Output terminals 48v and 491 connect through points a and b to the control and apparatus circuits F and G, as in Fig. 1.

The operation of this modified neutralizer is similar to that of Fig. l in that the winding of thereactor'53-withthe transformer I53 develops a voltage opposite. to that ofv the signal voltage so that the. inductive tool component of the signal voltage is neutralized.

The current compensator circuit D corresponds to the. arrangement of Fig. 1 with the exception that instead. of the potentiometer 54 being connected. directly in parallel with the resistor 53, transformer It! is interposed between these elements. The D circuit otherwise is connected and functions as in Fi 1*.

In Fig. 3,. there is shown the. modification of the invention as disclosed in my copending application above referred to. In this arrangement, a reactor coil I" is attached as by a support band IZLI to the yoke 45-6 of the welder. This coil. is placed in series: in the signal circuit con-- ductor 22 and hence, is. subjected to the same inductive effects as the signalconductor. By placing. the coil turns so as to develop voltage opposition to the" signal. voltage, the inductive efiect is neutralized.

The. signal conductors 22 are led from the tool directly to the step-up transformer I and thence to the rectifier unit I3I, including full wave'vacuum rectifier tube I32, reactor I33 and capacitors I34; Rectifier terminals I35, I36 are connected: to the: resistance load and to the E output terminals 48 and 49 and thence to F connectors a and b in the manner previously described.

The current. compensator circuitD is identical to that of Fig. 2 except that the reactor 53. and taps on current transformer secondary are omitted.

The term tool is used in. the specification and claims to denote any physical apparatus which by the. shape thereof imparts inductive voltage to an attached or included electric circult; Limitation of the word to welding equipment is not intended.

The specific neutralizing circuits disclosed, lend themselves especially to cooperative use with a voltage control system of the type shown in the drawing. However, adaptation to other uses may be made and limitation to-the particular showing of the drawing is not intended, other than may be required by the claims as hereto appended.

What is claimed is:

1. Control apparatus comprising in combination, a source of alternating electric current, a current-consuming device connected to said source, control means including voltage comparator means conditioned by a pre-change voltage and susceptible to actuation by comparison of pie-change voltage with post-change voltage at said device due to change of electrical resistance in the work circuit thereof, said device normally inducing in said control means a voltage due' to the passage of current in said work circuit, and neutralizing means connected tosaid control means for neutralizing the inductive voltage due to the current how in said work circuit, said neutralizing means comprising a resistor connected to said control means, and variable voltage means connected to said resistor adapted to impart a voltage therethrough counter to the induced voltage in said control means due to current flow in the Work circuit.

2. Control apparatus comprising in combination, a Source of alternating electric current, a current-consuming device connected to said source, control means including voltage comparator means conditioned by a pre-change voltage and susceptible to actuation by comparison of pre-change voltage with post-change voltage at said device due to change of electrical resistance in the Work circuit thereof, said device normally inducing in said control means a voltage due to the passage of current in said work circuit, and neutralizing means connected to said control means for neutralizing the inductive voltage due to the current flow in said work circuit, said neutralizing means comprising a resistor connected to said control means, and an inductive reactor connected to both said load circuit and said resistor for imparting to the control means across said resistor a voltage variable with the induced voltage but 180 out of phase therewith, whereby the control voltage is corrected for the induced voltage.

3. Control apparatus comprising in combination, a source of alternating electric current, a current-consuming device connected to said source, control means including voltage comparator means conditioned by a pro-change voltage and susceptible to actuation by comparison of pre-change voltage with post-change voltage at said device due to change of electrical resistance in the work circuit thereof, said device normally inducing in said control means a voltage due to the passage of current in said work circuit, and neutralizing means connected to said control means for neutralizing the inductive voltage due to the current flow in said work circuit, said neutralizing means comprising a resistor connected to said control means, variable voltage means connected to said resistor adapted to impart a voltage therethrough counter to the induced voltage in said control means due to current flow in the Work circuit, and means for maintaining a fixed ratio between the induced voltage and the neutralizing voltage.

4. In Weld control apparatus, a source of alternating electric current, a welding tool yoke, electrodes attached to said yoke and adapted to receive electric current through said yoke from said current source for effecting welds, circuit connections between said source and electrodes through said yoke, control means susceptible to voltage change at the electrodes due to resistance change at weld fusion, circuit connections between said electrodes and control means, and neutralizing means connected to said electrodecontrol means connections for neutralizing the voltage at the electrodes due to the inductance of the tool yoke, said neutralizing means comprising an electrical transformation device connected to said electrode-control means connections and to said source-electrode connections for transmitting to said control means a voltage opposed and equal to the inductive voltage transmitted from the electrodes to the control means.

5. Control apparatus comprising in combination, a source of alternating electric current, a current-consuming device connected to said source, control means including voltage comparator means conditioned by a pre-change voltage and susceptible to actuation by comparison of pre-change voltage with post-change voltage at said device due to change of electrical resistance in the work circuit thereof, said device normally inducing in said control means a voltage due to the passage of current in said work circuit, and neutralizing means connected to said control means for neutralizing the inductive voltage due to the current flow in said work circuit, said neutralizing means comprising a resistor connected to said control means, variable voltage means connected to said resistor adapted to impart a voltage therethrough counter to the induced voltage in said control means due to current flow in the work circuit, means for maintaining a fixed ratio between the induced voltage and the neutralizing voltage, and means for compensating in said voltage control means for voltage change therein due to current change in the load circuit.

6. In control apparatus, a source of alternating electric current, a current-consuming device electrically connected to said source, control means electrically connected to said device and operable by voltage change in the work circuit thereof, said device normally developing an induced voltage in said control means, a current transformer connected to the source-device circuit, and plural voltage transforming means connected from said transformer to said device-control circuit, one of said transforming means transmitting a voltage for neutralization of voltage in said device-control circuit due to induced voltage, and another of said transforming means compensating in said device-control source for current changes in said source-device circuit.

'7. In control apparatus, a source of alternating electric current, a current-consuming device connected to said source, control means electrically connected to said device and operable by voltage change in the Work circuit thereof, said device normally developing an induced voltage in said control means, and neutralizing means connected to said control means for neutralizing the induced voltage of said device in said control means, said neutralizing means comprising a transformer connected to said control means for impressing a voltage thereon counter to the induced voltage at the control means received from said device, and an inductive coupling between the device and transformer for transmitting current change due to inductive voltage at the de vice to said transformer.

8. In weld control apparatus, a source of alternating electric current, a welding tool yoke, electrodes attached to said yoke, circuit connections between said source and electrodes through sections of said yoke, control means including voltage comparator means conditioned by pre-fusion voltage and susceptible to actuation through comparison of pro-fusion voltage with post-fusion voltage at the electrodes, the change of voltage being due to change of resistance upon occurrence of fusion at welding, circuit connections between said electrodes and control means subject to inductive effects in said yoke, and neutralizing means connected to said electrodecontrol means connections for neutralizing the voltage formed at the electrodes due to the inductance of the tool yoke, said neutralizing means comprising a transformer electrically connected to the control means and electrodes, induction means between said voltage transformer and said source-electrode circuit, and a variable reactor connected to the source-electrode connections and transformer whereby voltage approximately equal to and opposed to the inductive voltage oi-the tool yokeis transmitted to said control means.

9. In a-weld control apparatus, asource of alternating electric current, a welding tool comprising a yoke and electrodes supported by said yol:e,:circuit connections between the source and electrodes passing through said yoke, switch means for establishing-current supply to the electrodes at a substantially constant electrode voltage priorto fusion, control means including voltage comparator means conditioned by prefusion voltage and susceptible to actuation through comparison of pro-fusion voltage with post-fusion voltage connected to the switch means and electrodes and subject to inductive eifects of current in said yoke and acting at the voltage change point due to weld fusion to actuate said switch means, a current transformer in said source-electrodecircuit, a voltage transformer in said electrode-control means circuit, induction means between said voltage trans- .former and said source-electrode circuit, an inductive reactor electrically connected between said transformers, the winding relationship of the transformer-reactor-transformer system be ing such as to develop at the voltage transformer a voltage approximately equal and opposite to the induced voltage developed in the electrodecontrol means circuit'by voltage change in the tool yoke, ,a rectifier connected to said current transformer, and electronic means operated by said rectified voltage for changing the control means voltage to compensate for current change in the source-electrode circuit.

10. In control apparatus in combination, a current-consuming device having :a current supply circuit and a work circuit, a control circuit for measuring'voltage change in a part of said Work circuit and subject to aninduced voltage due to current flowing in said work circuit which would objectionably influence the desired signal due to voltage change, a compensating circuit associated with said current supply circuit and producing an inductive effect in opposition to that induced in the control circuit by the work circuit, and means associating said compensating circuit and said control circuit insuch manner as to impose the opposed inductive eiiect of the compensating circuit on the control circuit.

11. .In control apparatus in combination, a current-consuming device having a current supply circuit and'a work circuit, a control circuit for measuring voltage change in a part of said work circuit and subject to an induced voltage due to current flowing in said work circuit which would objectionably influence the desired signal due to voltage change, a compensating circuit associated with said current supply circuit and produc- 12 ing an inductive effect in opposition to that induced in the control circuit by the work circuit, and means associating said compensating circuit and said control circuit in such manner as "to impose the opposed inductive efiect of the compensating circuit on the control circuit, said associating means including a reactor coil in said compensating circuit, and circuit connections between said reactor coil and said control circuit.

12. In control apparatus in combinatioma cur- :rent-consuming device having a current supply vcircuit and a work circuit, a control circuit for measuring voltage change in a part of said work circuit and subject to an induced voltage due to current flowing in said work, circuit which would objectionably influence the desired signal due to voltage change, a compensating circuit associated with said current supply circuit and producing an inductive effect in opposition to that inducedin the control circuit by the work circuit,

.and means associating said compensating circuit and said control circuit in such manner as to impose the opposed inductive effect of the com- :pensating circuit on the control circuit, said associating means including a reactor coil in said compensating circuit, and a shunt circuit connected with both said compensating circuit reactor coiland said control circuit.

13. In control apparatus in combination, a current-consuming device having a current supply circuit and a work circuit, a control circuit for measuring voltage change in a part of said worl: circuit and subject to an induced voltage due to currentiiowing in said work circuit which would objectionably influence the desired signal due to voltage change, a compensating circuit associated with said current supply circuit and producing an inductive effect in opposition to that induced in the control circuit by the work circuit, and means associating said compensating circuit and said control circuit in such manner as to impose the opposed inductive eilfect of the compensating circuit on the control circuit, said associating means including a reactor coil in said compensating circuit, and a shunt circuit connected with said reactor coil and having an inductive connection as by a transformer with said control circuit.

EDWIN M. CALLENDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,622,702 Weed June 5, 1928 1,959,690 Roth May 22, 1934 1,991,414 Rees Feb. 19, 1935 2,346,645 Bayles Apr. 18, 1944 2,370,009 Clark et a1. Feb. 20, 1945 

